probabilistic termination of transparent bounces
From Blender Manual &gt; Light Paths &gt; Bounce Control

Light paths are terminated probabilistically when specifying a minimum
number of light bounces lower than the maximum. In that case, paths
longer than minimum will be randomly stopped when they are expected to
contribute less light to the image. This will still converge to the same image, but renders faster while possibly being noisier.



Related thread on BlenderArtists.org